Changes in transcripts encoding the [alpha]1-and [beta]-subunits of
the calcium channel in rat myometrium during pregnancy and effects of
progesterone.
Tezuka, Naohiro, Mariam Ali, Kristof Chwalisz, and Robert E. Garfield.
Division of Reproductive Sciences, Department of Obstetrics and
Gynecology, University of Texas Medical Branch, Galveston, Texas
77555 and Research Laboratories of Schering AG, Berlin, Germany
APStracts 2:0189C, 1995.
Extracellular Ca2+ is normally required for myometrial cells to
contract. Ca2+ enters muscle cells mainly through voltage-dependent
Ca2+ channels (VDCCs) that open in response to action potentials. The
synthesis of myometrial VDCCs may change during pregnancy to alter
excitation-contraction coupling. We investigated the mRNA levels for
the [alpha]1-and [beta]-subunits of the L-type VDCC in rat myometrium
to determine whether alterations are associated with term or preterm
labor. RNA isolated from myometrial tissues was analyzed by reverse
transcription-polymerase chain reaction (RT-PCR) using specific
primers designed according to the published sequences of the VDCC
subunits. From pregnant rat myometrium, two distinct PCR products
were obtained for the [alpha]1-subunit: one of the expected size at
372 base pairs (bp) and a smaller at 339bp. Sequence analysis of the
larger product revealed a 99.5% or 88% sequence homology between rat
myometrium and rat aorta or rabbit heart, respectively, and the
smaller product had an identical sequence with a 33 bp deletion. The
two [alpha]1 products followed the same trend throughout pregnancy.
VDCC [alpha]1 mRNA levels increased gradually to 6.9 - fold just
prior to labor on day 22, but decreased during labor. However, the b
-subunit mRNA level increased sharply on day 22 and then also declined
during labor. Progesterone treatment from day 19 to day 22 inhibited
term delivery and prevented the significant increase in [alpha]1 mRNA
levels. In contrast, antiprogestin (onapristone, ZK98.299) treatment
on day 17 caused a statistically significant increase in the [alpha]1
and [beta] VDCC subunit mRNA after respectively 8 and 15 hours then a
decrease during preterm labor at 24 hours. We conclude that mRNA
levels for the VDCC subunits increase prior to term and preterm
labor, but decline during periods when VDCCs are likely at their
peaks. The increase in levels of mRNA for VDCC likely reflect changes
in expression of VDCCs during periods of term and preterm labor which
may facilitate uterine contractility required for this process.
Progesterone withdrawal or blockade appears to be responsible for
regulating levels of mRNA for VDCC in the myometrium in preparation
for labor.
Received 29 August 1994; accepted in final form 21 April 1995.
APS Manuscript Number C504-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 16 May 1995.